The present invention relates to a method of refining molten steel such that electrodes are submerged in a slag layer of the molten steel and generate arcs within the slag to refine the molten steel in accordance with the submerged arc heating method.
If molten steel is refined in a steel converter and is then arc-heated by arc heating process, the temperature of molten steel flowing out of the steel converter can be decreased. Therefore, losses of additives and consumption of refractory material of the converter can be decreased and the yield of the molten steel from the converter can be increased. In practice, when the arc process is performed, carbon electrodes 6 are preferably submerged in a slag 4 on the molten steel 2, and an arc is generated between the electrodes 6 and the molten steel 2 while the electrodes 6 are substantially surrounded with the slag 4 (namely submerged arc heating), as shown in FIG. 1. When the slag 4 is not present, an arc jumps between the electrodes and the ladle furnace wall or between the electrodes and a gas blow lance, so that the refractory materials are locally damaged. In addition, the refractory furnace wall is heated by heat radiated from an arc column. In general, the thickness of the slag layer is set to l+50 mm, where 50 mm is a margin and l is given as follows: ##EQU1## where V.sub.2 and I.sub.2 : the secondary line-to-line voltage and the secondary current
.alpha.: the power factor PA1 K: the potential difference (40 V) between the anode and cathode PA1 R: the circuit resistance
In general, Ar gas is introduced into the molten steel to stir the molten steel so as to keep the molten steel and the slag at a uniform temperature in the arc process. By stirring, the surface of the molten steel is undulated bringing the molten steel into contact with the electrode, thereby damaging the electrode. In addition, carbon from the electrode enters the molten steel directly or through the slag. As a result, the carbon content [C] of the molten steel is increased.
FIG. 2 shows the relationship between the slag thickness and increase speed of the carbon content [C] when 250 tons of molten steel is heated for one heat or charge at a heating rate of 4.degree. to 5.degree. C./min. When the slag thickness and the arc length are increased, the increase speed of the carbon content [C] can be decreased. However, as apparent from the graph in FIG. 2, even if the slag thickness is increased, an increase in the carbon content [C] cannot be completely prevented. In addition, the amount of flux used and the amount of power consumed are increased, resulting in inconvenience.
For example, when the slag thickness is 100 mm, the carbon content of the molten steel is increased by 0.005% when arc heating is performed for 10 minutes. When such a change in carbon content occurs, the control of the carbon content in a low-carbon steel is difficult. For this reason, in low-carbon steel, especially, in ultra-low-carbon steel having a carbon content of less than 0.015%, the arc process is not conventionally utilized. When low-carbon steel is subjected to RH degassing, the temperature of molten steel flowing out of a steel converter must be not less than, e.g., 1,700.degree. C. Various problems are presented since this temperature is very high.
When molten steel is heated by the arc process, the time for the arc process must be changed in accordance with the end of the refinement of the molten steel by the converter and the commencement of the continuous casting. However, according to the conventional steel heating technique, the heating rate is not considered, so a sufficient and complete match with other processes cannot always be established.
In order to prevent the slag on the molten steel from being excessively heated and to keep the molten steel and slag at a predetermined temperature when the molten steel is arc-heated, a gas is introduced into the molten steel to stir the steel together with the slag. In this manner, the slag must be prevented from being heated to a high temperature. However, in this case, the molten steel is often brought into contact with the electrode due to the undulation of the steel by the introduction of gas. When this occurs, the arc becomes unstable and the electrode is excessively worn. Carbon is picked up from the carbon electrode by the molten steel. When the molten steel is desulfurized during the arc process, the molten steel and the slag are stirred after arc heating is performed so that the slag reacts with the molten steel to desulfurize the molten steel. In this case, the molten steel and the slag must be vigorously stirred. However, according to this conventional stirring method, satisfactory results cannot always be obtained.